1. Field of the Invention
The present invention relates to an electron gun for a cathode ray tube (CRT).
2. Description of the Related Art
FIG. 4 illustrates a conventional electron gun and associated components that form three electrodes, showing a cross-sectional view when the electron gun is cut by a Y-Z plane and seen in an X-direction of FIG. 6.
FIG. 4 illustrates a cathode KG for green, a G1 electrode 1 with a G1 strap 1a, a G2 electrode 2, a G3 electrode 3, and a G4 electrode 4 when a measurement element 10 is inserted into the electron gun. Other electrodes following the G4 electrode are not shown. A supporting member 5 formed of hermetic glass firmly holds three cathodes KR, KG, and KB spaced apart predetermined intervals. Another supporting member 6 supports stems that individually supply voltages received from an external electrical circuit to the electrodes. A bead glass 7 securely holds all of the aforementioned components 1a, 2, 3, 4, and 6 in place. For example, the measurement element 10 shown in FIG. 4 is used to measure the distance (referred to as G1-2 distance) between the G1 electrode 1 and the G2 electrode 2 and the distance (referred to as G1-K distance) between the G1 electrode 1 and the cathode KG.
FIG. 5 is a rear view of electron guns of the conventional CRT as seen from a Z-axis of FIG. 6.
Referring to FIG. 5, the conventional CRT has the cathode KR for red, the cathode KG for green, and the cathode KB for blue. The supporting member 5 is formed with holes spaced a predetermined distance apart. The cathodes KR, KG, and KB are inserted into the holes and welded.
The conventional CRT of the aforementioned construction operates as follows:
Each electron beam of the respective electron gun passes through holes formed in the respective electrodes. The holes formed in the respective electrodes are formed progressively large as the electrodes are away from the cathode or are greater than a predetermined size. Electrodes that follow the G1-G4 electrodes are not shown but their beam-passing holes are of the same as or greater than that of the G4 electrode.
In order to achieve highly focussed images, the holes are so designed that the crossover diameter of electron beams is as small as possible. For this purpose, the three electrodes are formed with small diameters to minimize crossover of the electron beams emitted from the cathode. In order to achieve even smaller diameter of electron beams that have subjected to crossover, a focussing electrode is designed to have a beam-passing hole with as large a diameter as possible. The small diameter hole provides a main electronic lens having less lens aberration, suitable for good focussing.
The electron gun of the conventional CRT has beam-passing holes of the aforementioned structure.
The measurement element 10 shown in FIG. 4 will now be described. The assembly accuracy of a cathode determines the cut-off voltage that is an important characteristic of an electron gun used for a CRT. Therefore, the cathode must be assembled with an accuracy of less than several microns.
The electrodes for each electron beam of a conventional CRT have beam-passing holes progressively larger than that of the G1 electrode 1 as the electrodes are away from the cathode. The measurement element 10 is inserted into the beam-passing hole of an electrode farthest away from the cathode with out difficulty. The measurement element 10 is used to measure the G1-2 distance of the CRT. The position of the cathode is calculated based on the G1-2 distance, thereby determining the G1-K of the CRT. Then, the cathode is welded to the supporting member 5 at a calculated position.
Thus, even if the G1-2 distance is slightly different for red, green, and blue cathodes, the positions of the respective cathodes can be adjusted individually so that the cut-off voltage of the three cathodes can be adjusted to substantially the same value. Further, when the CRT is tested after assembly, the nozzle 10 can be used again to detect defective spaces between the electrodes and the cathode. This prevents detective CRTs from being shipped out.
As described above, the conventional CRT of the aforementioned configuration facilitates insertion of the measurement element 10 to the cathode without difficulty. The configuration allows adjustment of the cut-off voltage of the respective cathodes and test after assembly.
Recently, in order to improve the focussing performance, an electron gun used for a CRT has the G2 electrode having a smaller diameter than the G1 electrode or has an additional electrode (referred to as GM electrode hereinafter) having a small beam-passing hole. However, such a configuration of beam-passing hole does not allow the conventional measurement element to reach the G1 electrode depending on the size of the smallest beam-passing hole, making it difficult to measure the G1-2 distance. This necessitates the assembly of cathode only to an initially designed position. Thus, not only the cut-off voltage is more prone to a change in G1-2 distance but also the electron gun cannot be tested after assembly.
The present invention was made in view of the aforementioned drawbacks of the conventional CRT.
An object of the invention is to provide an electron gun for use in a CRT that can alleviate defective rate of CRT production while still maintain existing performance and reliability.
A cathode ray tube has a set of three electron guns. Each electron gun includes at least a cathode, a G1 electrode, a G2 electrode aligned in line in this order, the G1 electrode and the G2 electrode having a first and a second hole formed therein, respectively, through which an electron beam emitted from the cathode passes. The electron gun also includes a GM electrode placed between the G1 electrode and the G2 electrode. The GM electrode has a third hole through which the electron beam passes, the third hole being smaller than the first hole and/or the second hole. A supporting member supports the cathode in position. The supporting member and the G1 electrode are formed with a fourth and a fifth hole through which a measurement element is inserted to measure a distance between the G1 electrode and the G2 electrode.
The G2 electrode may be formed with a sixth hole through which the measurement element is inserted to measure a distance between the GM electrode and the G2 electrode.
The fourth to sixth holes are in line with one another.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.